Whenever I open my first program the icons move slightly to the right and then when I close all the programs the icons on the left handside will move left slightly.
I've tried different themes and it does it with all of them so is there something I can change in the panel settings?
I've tried to take a screenshot of before and after but it's pretty difficult, you can just about see that the Linux Mint icon is slightly further away from the left handside of the monitor than in the 'Before' screenshot:
Before:

After:




